In my previous U.S. Pat. No. 3,706,344, there is disclosed a perforating gun which is detonated by dropping a bar down the tubing string. The bar impacts against a firing head which detonates the shaped charges of the gun. Once the bar has been dropped down the tubing, the system is committed to proceed with the final completion steps.
Often it is desirable to install the perforating gun while various workover equipment is on hand, and to complete the well at some subsequent date. This effects a considerable savings in money and also provides a means for isolating the hydrocarbon containing zone so that the well can remain dormant until the subsequent completion date arrives. During this time, there is always a danger of some objects being accidently dropped downhole and inadvertently firing the gun and completing the well without the necessary surface equipment and technical people being on hand. Moreover, during a long interval of time, it is possible for debris to collect above the firing head so that when the bar is subsequently dropped in order to attempt completing the well, the overlying debris prevents the bar from contacting the firing head, and the perforating gun cannot be fired.
There are many highly deviated boreholes slanted towards the horizontal to an extent which precludes the use of gravity induced impact for firing a perforating gun. In this instance, some means, other than a bar, must be employed for detonating the gun. This is especially so in offshore rigs where multiple boreholes are formed by slanting each of the wells away from a single drilling platform, thereby radially spacing the wells respective to one another.
In my previous U.S. Pat. Nos. 3,966,236 and 4,066,282, it is pointed out that one can often foresee that a tool string located on the lower marginal end of a tubing string must be subsequently disconnected from the remainder of the string and abandoned downhole in the wellbore. It is especially important when running tool strings into expensive offshore wells to be able to assure everyone concerned that should the string become stuck downhole, that provisions are included for releasing the stuck part of the tool string.
As pointed out in my previously issued U.S. Pat. No. 3,706,344, it is advantageous to be able to complete a well while the tubing string is open to ambient so that a maximum pressure differential is achieved across the production formation instantaneous with the perforation thereof so that the debris is immediately flushed out of the newly completed formation, thereby avoiding any contamination of the production zone with extraneous material.
It would be desirable to be able to circulate fluid downhole and across the firing head of a gun in order to wash debris therefrom. It would also be desirable to be able to open the main vent assembly of the tubing simultaneously with the actuation of the firing head. It would further be desirable to protect the firing head of the perforating gun prior to detonation thereof. Moreover, several heretofore unknown advantages would be realized if one were able to increase and then decrease the circulation rate of the lower borehole annulus, and to be able to close off the lower tubing string, or to open the lower tubing string at any time prior to detonation of the gun. It would furthermore be desirable to be able to fire the gun while the tubing string is vented to ambient at the surface of the ground.
Apparatus and method which enable the above desirable manipulations to be performed while completing a cased wellbore is the subject of the present invention.